1. Field of the Invention
This invention relates to a dry etching method employed in the preparation of semiconductor devices. More particularly, it relates to a dry etching method for etching a layer of an aluminum-based material with improved resist selectively.
2. Description of the Related Art
As metallization materials for semiconductor devices, aluminum (Al), an All-Si alloy containing 1 to 2 wt % of Si besides Al and an Al-Si-Cu alloy, further containing 0.5 to 1 wt % of copper (Cu) for combatting stress-migration, are employed extensively.
Dry etching of the layer of the Al-based material is generally performed using chlorine-based gases. Typically of these is a BCl.sub.3 /Cl.sub.2 mixed gas as disclosed in JP Patent KOKOKU Publication No. 59/22374 (1984). Since the chemical species functioning as a main etchant during etching of the layer of the Al-based material is Cl* (chlorine radicals), it is inherently sufficient to use only Cl.sub.2. However, a native oxide is present in usual on the surface of the layer of the Al-based material and, unless this oxide is reduced, etching does not proceed smoothly. On the other hand, etching proceeds only isotropically if only Cl* are used, so that ions need to be present in some form or other to achieve anisotropic etching. BCl.sub.3 is added in this consideration. That is, since BCl.sub.3 yields BCl.sub.x, a chemical species exhibiting a strong reducing action in the plasma, reduces the native oxide film on the surface of the layer of the Al-bases material to cause the etching reaction to proceed smoothly. On the other hand, since ions such as BCl.sub.x.sup.+ yielded from BCl.sub.3 promotes forward sputtering of a resist mask to cause carbonaceous reaction products, such as CCl.sub.x, to be deposited on the pattern sidewall, anisotropic etching is enabled under such sidewall protection effect by the deposits.
However, for achieving anisotropic etching by the above mentioned BCl.sub.3 /Cl.sub.2 mixed gas, it is presupposed that a moderately high ion incident energy be used to produce forward sputtering of the resist mask, as mentioned previously. For this reason, resist selectivity is essentially low and was heretofore of the order of 2 at most. Recently, in keeping up with the tendency towards higher integration degree of semiconductor devices and more complicated device arrangement, the wafer surface step difference tends to be increased, so that the necessity for significant overetching is correspondingly increased. In this consideration, such low selectivity may prove to be a hindrance in carrying out the process in future.
As a concept for overcoming such low selectivity, it has hitherto been contemplated to intensify sidewall protection effects by the etching reaction products to reduce the ion incident energy required for anisotropic etching to inhibit excess sputtering of the resist mask.
For example, in Digest of Papers, 1990 3rd. Micro Process Conference, B-6-3, a report is made of an example of effecting anisotropic etching of an Al-Si-Cu layer using a BBr.sub.3 /Cl.sub.2 mixed gas. With this mixed gas, a main etchant for the Al-Si-Cu layer is furnished from Cl.sub.2, while Br from BBr.sub.3 is combined with carbon atoms contained in the resist material to yield CBr.sub.x which plays the role not only of contributing to achievement of shape anisotropy by being deposited on the pattern sidewall, but of protecting the resist mask from ion sputtering on the resist mask surface to lower the etchrate. The result is the improvement in anisotropy and resist selectivity.
The present Assignee has also proposed a technique of etching the layer of the Al-based material using a gas containing hydrogen as a component element in a molecule thereof, referred to hereinafter as an H-based gas. If a BCl.sub.3 /Cl.sub.2 /HCl mixed gas, for example, is used, deposition of a resist decomposition product having the composition of CCl.sub.x H.sub.y is promoted on the pattern sidewall to improve shape anisotropy. Since the deposition is promoted in this manner, the bias power necessary for anisotropic etching is correspondingly reduced for improving resist selectivity. In addition, if the underlying oxide layer is exposed during overetching and sputtered so that oxygen functioning as an etchant for the resist mask is released into the etching system, such oxygen is captured by H-based active species and stabilized as OH, so that resist selectivity may similarly be improved in this manner.